The present invention relates to a system for a four-wheel drive vehicle for automatically changing the transmission system of the vehicle from two-wheel drive system to four-wheel drive system or vice versa according to driving conditions.
In a well-known four-wheel drive vehicle, a power transmission system for the two-wheel driving is selectively converted to the four-wheel driving system by engaging a clutch which is manually operated by a select lever.
Such a four-wheel drive vehicle is preferably driven by the two-wheel driving system on paved dry surfaces. If all four-wheels are driven on such the dry surface, an amount of tire scraping occurs because of slight differences in effective wheel radii caused by inevitable differences in tire inflation, tread wear or variation in loading. In addition, when the vehicle negotiates corners, braking phenomenon called "tight corner braking" will occur at a sharp corner. This is caused by the reason that the front wheels run through an arc of greater radius than that of the rear wheels and therefore tend to rotate faster than the rear wheels. This will result in increase of tire wear and fuel consumption and decrease of driveability.